Scientists have painstakingly measured 2.7 billion-year-old raindrop fossils from South Africa. The size of the ancient droplets tells the story of how the earth was teeming with microbes when it should have been frozen solid.

Between two and four billion years ago, the sun was 30 percent dimmer than it is today, so really, the earth should have been frozen solid. But we have evidence of rivers and ocean sediments from back then, so something was making the planet warm. The ancient raindrops revealed that it was greenhouse gases—they increased atmospheric pressure, heating up our world.

Here's how the researchers figured all that out.

Hairspray, Latex and Stairwells

A scientist can't just use a barometer to measure the atmospheric pressure of a fossilized droplet. Instead, Roger Buick and Jelte Harnmeijer poured latex over the 2.7 billion-year-old raindrop impressions, which were petrified in volcanic ash found in South Africa. They shipped the latex peels to their lab at the University of Washington in Seattle. There, a high-precision laser scanner measured the droplets.

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The size of raindrop impressions gets larger as the velocity of the droplet increases. Higher atmospheric pressure will slow that velocity. The composition of material onto which the raindrops fall also effects the size of the droplets. Previous research showed that raindrops on earth don't exceed a quarter inch in diameter, so the researchers figured that was the largest any of the ancient raindrops could have been, regardless of atmospheric pressure.

In today's atmosphere, the largest raindrops fall at about 30 feet per second. But if the ancient atmosphere was thicker, the speed would have been lower, and the maximum size of imprints left behind would be smaller.

To create a present-day comparison, two other researchers released water droplets of varying sizes 90 feet down an open stairwell onto volcanic ash collected from Hawaii and Iceland. Sanjay Som, lead author on the study, worked with Peter Polivka to coat the raindrop sculptures with liquid plastic and hair spray. This kept the ash molds intact, and then the laser could scan the impressions.

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Comparing the two showed that, if the biggest imprints were formed by the largest raindrops, the atmospheric pressure 2.7 billion years ago could have been no more than twice what it is today. And since the largest raindrops were actually few and far between, the pressure was probably the same, or possibly lower. Without a hot sun to create the pressure and heat, they concluded, a buildup of greenhouse gases in the atmosphere was keeping Earth toasty.

Life on Other Planets

If atmospheric pressure allowed life on a planet that should have been frozen, Som said in a statement, scientists could better estimate the probability of life on the exoplanets currently being discovered.

Setting limits on atmospheric pressure is the first step towards understanding what the atmospheric composition was then. Knowing this will double the known data points that we have for comparison to exoplanets that might support life. Today's Earth and the ancient Earth are like two different planets.

Sometimes it sounds really freaking fun to be a scientist. The raindrop fossil test results appear in the March 28 issue of Nature. [Nature]